In recent years, as an oscillator used for a time reference of an electronic apparatus and the like, an oscillator having a small size and a small CI (crystal impedance) value has been required in accordance with miniaturization of the electronic apparatus in which the oscillator is used. For example, in the case of a tuning fork type crystal oscillator, as an oscillator responding to this requirement, there is conventionally known an oscillator, such as shown in FIG. 7A and FIG. 7B (for example, see Japanese Patent Application Laid-Open No. 2002-76827).
As shown in FIG. 7A, a tuning fork type crystal oscillator 110 has tines 112 and 113, and grooves 112a and 113a are formed on the front and rear surfaces of the tines 112 and 113 along the lengthwise direction of the tines. As a result, as shown in FIG. 7B, the tines 112 and 113 have an H shaped cross section, respectively. Here, such a tuning fork type crystal oscillator having an H shaped cross section has a characteristic that even when the size of the oscillator is reduced as compared with before, the electromechanical conversion coefficient can be increased and thereby the CI value or an equivalent series resistance can be suppressed low.
In the tuning fork type crystal oscillator 110 having such an H shaped cross section, the tines 112 and 113 are caused to vibrate, when a voltage is applied from the outside. The vibration causes a drive current to flow through the series resistance part of the equivalent circuit. A groove electrode is formed in each of the grooves 112a and 113a which are formed on the front and rear surfaces of the tines 112 and 113, and a side surface electrode is formed on each of side surfaces (surfaces on which the grooves 112a and 113a are not formed) 112b and 113b of the tines 112 and 113. Then, when a voltage is applied, an electric field is produced between the groove electrode and the side surface electrode, to thereby cause the part of the tines 112 and 113 on the left and right side of the groove to be expanded and contracted in the directions opposite to each other due to the piezo-electric effect. As a result, flex vibrations in the directions opposite to each other are produced in the tines 112 and 113, so as to enhance the Q value of the oscillator. Thereby, a current is made to flow between the side surface electrode and the groove electrode.
However, the current needs to be supplied to the side surface electrode and the groove electrode from the outside. Specifically, the current is supplied to the groove electrode and the side surface electrode from the outside via a base electrode provided for a base 111 of the tuning for k type crystal oscillator 110. For this reason, connecting electrodes for connecting the base electrode with the groove electrode and the side surface electrode are needed. Among these connecting electrodes, a connecting electrode for groove electrode for connecting the base electrode with the groove electrode is arranged on the base surface 111c in FIG. 7A. Further, a groove/side surface connecting electrode for connecting the groove electrode with the side surface electrode is arranged, for example, on the base surface 111c and the tine surface 112c. 
In such a tuning fork type crystal oscillator 110 having the H shaped cross section, it is possible to shorten the distance between the groove electrode of the grooves 112a and 113a, and the side surface electrode of the side surfaces 112b and 113b, and to apply an electric field in the direction almost in parallel with the x axis of the quartz, and to enhance the electromechanical conversion efficiency based on the piezo-electric effect. Thus, when a drive voltage is applied between the electrodes, the tuning for k type crystal oscillator 110 is easily deformed so as to allow the current to flow. That is, the tuning fork type crystal oscillator 110 having the H shaped cross section is more advantageous than a conventional tuning fork type crystal oscillator without grooves in reducing the CI value.
However, in connection with the tuning fork type crystal oscillator 110 having the H shaped cross section, for example, there is a case where a small-sized oscillator having a resonance frequency of 32.768 kHz is required. In accordance with this requirement, the width of the tines 112 and 113 shown in FIG. 7A is reduced to, for example, 0.1 mm, and the width of the grooves 112a and 113a is reduced to, for example, about 0.07 mm. Accordingly, the width W of the region (hatched region in FIG. 7A) on the tine surface 112c, in which region the base electrode and side surface electrode are connected with each other, is limited to, for example, 0.015 mm. Here, the width of the groove/side surface connecting electrode which is to be arranged on the tine surface 112c needs to be at least about 0.01 mm. This results in that the gap between the groove/side surface connecting electrode and the groove electrode can be allowed to only 0.005 mm. In view of an error produced in the actual manufacturing process, this results in problems that these electrodes are more likely to come into contact with each other, and to cause other short-circuits, thereby forming a cause of defects of the oscillator, and that when the oscillator is manufactured so as to eliminate such cause of defects, manufacturing costs of the oscillator are significantly increased.
Then, in order to improve such problems, there is proposed a tuning fork type crystal oscillator as disclosed in Japanese Patent Application Laid-Open No. 2003-87090. This tuning fork type crystal oscillator 100 is shown in FIG. 8A to FIG. 8C.
In FIG. 8A, tines 120 and 130 are upwardly projected from a base 140. Grooves 120a and 130a are formed on the front and rear surfaces of the tines 120 and 130, respectively. As a result, the cross section of the tines 120 and 130 is formed approximately into an H shape, as shown in FIG. 8C. In the tuning fork type crystal oscillator having such H shaped cross section, base electrodes 140d and 140f are formed in the base 140, and groove electrodes 120d and 130d are formed in the grooves 120a and 130a which are formed in the tines 120 and 130. Further, as shown in FIG. 8C, side surface electrodes 120e and 130e are formed on the both side surfaces of the tines 120 and 130. Among the side surface electrodes, the side surface electrodes 130e arranged on the side surfaces of the tine 130 are connected to the base electrodes 140d via a connection electrode 141 for side surface electrode shown in FIG. 8A, and the side surface electrodes 120e arranged on the side surfaces of the tine 120 are connected to the base electrode 140f via a connection electrode 142 for side surface electrode. On the other hand, the groove electrode 120d is connected to the base electrode 140d via a connection electrode 143 for groove electrode. Further, the groove electrode 130d is connected to the side surface electrode 120e of the tine 120 via a groove/side surface connecting electrode 144, and is eventually connected to the base electrode 140f through the side surface electrode 120e. 
An example of the size of the oscillator in FIG. 8A is shown in FIG. 8B. The width of the tines 120 and 130 is 0.1 mm. The length of the grooves 130a and 120a is 0.8 mm, and the width of the grooves 130a and 120a is 0.07 mm in most parts except the lower part of the grooves. In the lower part of the grooves 130a and 120a, i.e., in the region ranging from the lower end of the grooves 130a and 120a to a height position upper by 0.2 mm from the lower end, the width of the grooves is narrowed to 0.05 mm. That is, the width of the grooves 130a and 120a is abruptly changed from 0.07 mm to 0.05 mm at the height position upper by 0.2 mm from the lower end of the grooves 130a and 120a. 
As a result, as shown in FIG. 8B, on the left side of the lower part of the groove 120a on the tine 120 and 130, the width of which part is narrowed, a connecting electrode arrangement region S for arranging the groove/side surface connecting electrode 144 for connecting the groove electrode 120d with the side surface electrode 120e is formed.
In this way, in the tuning fork type crystal oscillator 100 shown in FIG. 8A, the width of the lower part of the groove is made narrower than the width of the part above the lower part, thereby making it possible to more widely secure the connecting electrode arrangement region S by the amount obtained by narrowing the width of the groove. Thus, by arranging the groove/side surface connecting electrode 144 in the region S, the occurrence of a short circuit between the groove/side surface connecting electrode 144 and the groove electrode 120d is suppressed.
However, the conventional tuning fork type crystal oscillator shown in FIG. 8A and FIG. 8B, in which the region for arranging the connecting electrode for connecting the groove electrode with the side surface electrode is secured by making the width of the groove formed into two steps in this way, has problems as described below, that is:
(1) The plane shape of the grooves 120a and 130a does not become a simple rectangular shape, but becomes a stepped shape. Thereby, when the grooves are formed by etching, the grooves actually tend to be formed in such a manner that the wide width part of the grooves is deeply etched, and that the narrow width part of the grooves is shallowly etched.
FIG. 9 is a sectional view of the groove 120a taken along the center line c shown in FIG. 8B. As shown in FIG. 9, the depth of the groove 120a is formed to become shallow from a position apart by 0.2 mm from the base 140 so as to correspond to the width of the groove, and a somewhat gradually stepped shape is formed. Here, the rising inclination of the step is not necessarily fixed, but tends to be changed within the range as shown by the dotted lines. This is due to the fact that at the place where the width of the groove is changed stepwise, the etching depth tends to follow the change of the width of the groove and to be abruptly changed, but in practice, the change in the etching depth is delayed under the influence of various factors affecting the etching rate, thereby causing significant variation in the inclination of the step in the depth. For this reason, there are not small number of cases where the three dimensional forms of the grooves in the left and right tines are not coincident with each other but slightly different from each other. As a result, there may be a case where the rigidity in the part close to the root of the left and right tines is slightly different for each of the left and right tines, so that the cancellation of vibrations in the left and right tines is not sufficiently performed, thereby causing the Q value to be lowered and the CI value to be increased.
(2) Next, in the narrow width parts in the grooves 130a and 120a, the distance between the groove electrodes 120d and 130d in the grooves, and the surface electrodes 120e and 130e on the side surfaces becomes large. This causes the strength of electric field (electric field in the electric axis direction of crystal) applied between the side surface and the groove in these parts to be reduced. In this case, the electric field strength is reduced in the parts close to the root of the tines 120 and 130, so that the driving force for deforming the tines is reduced, and the electromechanical conversion coefficient is lowered. This results in a disadvantage in driving the tuning fork type crystal oscillator 100 and in enabling a sufficient current to flow, and causes the CI value to be deteriorated.
Further, there is disclosed a tuning fork type crystal oscillator having grooves formed in tines, in Japanese Patent Application Laid-Open No. 2003-133895, in which in the part close to the lower end of the groove, the width of the groove is gradually reduced toward the lower end of the groove so as to allow the depth of the groove to be gradually changed in this part, thereby preventing disconnection of groove electrodes. However, the lower part of the groove, the width of which is gradually reduced, is not formed in the tine of the tuning fork type crystal oscillator, but is formed in the base, and hence, does not make any contribution to the formation of a region for arranging a connecting electrode for effecting connection between electrodes.
Accordingly, an object of the present invention is to improve the above described problems in the conventional tuning fork type crystal oscillator as exemplified in FIG. 8A to FIG. 8C. A further object of the present invention is to reduce the CI value as compared with the conventional one, while securing reliable connection to side surface electrodes in a small-sized tuning fork type crystal oscillator having grooves in the left and right tines.